I have a scheduled procedure that trigger mails to user using SMTP packages. It was working fine quite a period when we were with Oracle 10g and its not working now when we upgrade to oracle 11g. It throws the below error when we execute the procedure.

ORA-24247: network access denied by access control list (ACL)
ORA-06512: at "SYS.UTL_TCP", line 17
ORA-06512: at "SYS.UTL_TCP", line 246
ORA-06512: at "SYS.UTL_SMTP", line 127
ORA-06512: at "SYS.UTL_SMTP", line 150

Similar instructions to resolve the [url http://www.ora00600.com/scripts/11g/UTL_SMTP_ORA-06512.html]ORA-24247 issue. Bear in mind that granting it to PUBLIC means that every user can use it, whereas you might just want a specific user to be able to send mail.

After that it's common to get the [url http://www.ora00600.com/scripts/databaseconfig/ORA-29278.html]ORA-29278: SMTP transient error: 421 Service not available error message so check that out, too. You'll probably only get this if you haven't already configured some of the parameters required to send mail.